Defense Secretary Mark Esper said Monday that the United States has not made any decision to leave Iraq, shortly after the U.S. military said in a letter to Iraqi officials that U.S. forces would be relocating "to prepare for onward movement."

On Sunday, Iraqi lawmakers passed a nonbinding resolution calling for foreign troops to withdraw.

In the letter, released Monday, Marine Corps Brig. Gen. William Seely said that U.S. forces "respect your sovereign decision to order our departure." A U.S. military official confirmed the letter's authenticity.

But in a day of confusion, Esper said at the Pentagon that he could not confirm the authenticity of the letter because he had seen it only after it had been leaked. He said the letter "is inconsistent with where we are right now."

Gen. Mark Milley, chairman of the Joint Chiefs of Staff, said the letter "was a draft, it was a mistake, it was unsigned, it should not have been released."

Part of the letter was "poorly worded, implies withdrawal," Milley said. "That is not what's happening."

Esper said the U.S. remains committed to the campaign to defeat ISIS in the region.

Asked whether the U.S. military was prepared to strike Iranian cultural sites, as President Donald Trump has suggested, Esper said the military would "follow the laws of armed conflict." Asked whether that means no, because international law prohibits targeting such sites during war, Esper said: "That's the laws of armed conflict."

Esper said the U.S. remains prepared for any contingency with regard to Iran and that the message to Tehran is that the "ball remains in their court."

Meanwhile, the U.S. Defense Department has ordered an amphibious force of about 4,500 sailors and Marines to prepare to support Middle East operations, a defense official said Monday.

Iraqi caretaker Prime Minister Adel Abdul-Mahdi told U.S. Ambassador Matthew Tueller that the United States and Iraq needed to cooperate "to implement the withdrawal of foreign forces in accordance with the decision of the Iraqi parliament," according to a statement from the prime minister's office.

Briefing for lawmakers

U.S. senators are expected to be briefed by top U.S. officials Wednesday on the U.S. drone strike that killed top Iranian commander Qassem Soleimani last week.

Secretary of State Mike Pompeo, Esper, Milley and CIA Director Gina Haspel are expected to brief the lawmakers. House members are expected to attend a separate briefing session.

On the diplomatic front, at the invitation of Russian President Vladimir Putin, German Chancellor Angela Merkel will visit Moscow on Saturday with plans to discuss escalating tensions in the Middle East in the wake of Soleimani's killing, the Kremlin announced.

German Foreign Minister Heiko Maas is accompanying Merkel on the trip. Maas said Monday that Trump threatening Iraq with heavy sanctions if U.S. troops are forced to leave is "not very helpful."

In Washington, Rep. Adam Schiff, D-Calif., who chairs the House Intelligence Committee, said the House should hold open hearings on the escalating tensions between the United States and Iran.

"I think there should be open hearings on this subject," Schiff told the Washington Post. "The president has put us on a path where we may be at war with Iran. That requires the Congress to fully engage."

The U.N.'s top cultural body said Monday that the United States is a signatory to a 1972 treaty pledging not to attack cultural sites, two days after Trump tweeted that if Iran strikes "Americans, or American assets," the United States would target 52 Iranian sites, including some that are important to "Iranian culture."

"Those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD," he wrote.